Three Japanese nationals released by China admitted on their return to Tokyo that they had inadvertently entered a restricted military zone before their arrest.

In a press conference at Tokyo's Haneda airport, one of the men said they are "extremely sorry for causing all Japanese people great concern." The man also appealed for the release of a fourth Japanese who is still being held in China.

Executives of the Fujita construction company where the four worked said the men were well treated and interrogated in a "gentlemanly" way during their detention.

The four were arrested last week in northern Hebei province and accused of photographing a sensitive military site. The detentions worsened already high tensions between Japan and China over Japan's arrest of a Chinese fishing captain near disputed islands in the East China Sea.

China reacted furiously to the fishing captain's arrest, and Japan accused Beijing of imposing a "de facto" ban on exports to Japan of some critical materials.

Japanese Prime Minister Naoto Kan said Friday in parliament that he was concerned about China's military buildup and aggressive maritime activities. He urged Beijing to act as "a responsible member of the international community."

There were suspicions in Japan that the Fujita employees were arrested in retaliation for the fishing captain's detention.

But the French news agency quoted a Fujita manager Friday as saying the men were detained when a Chinese military vehicle passed by just as its employees were inadvertently photographing a gate at the military facility.

The men were in China in connection with a project to dispose of chemical weapons left behind by Japanese military forces after World War II.